MF power generators, also called MF power supply units, for MF plasma processes or for induction heating usually include an AC/DC converter that converts an a.c. mains voltage into a regulated or an unregulated d.c. voltage. A particular current source characteristic can be realized by way of downstream inductances. The MF power generator also includes an inverter connected to the AC/DC converter, and an output network situated downstream of the inverter. The output network usually consists of an oscillating circuit that can be a parallel or a serial oscillating circuit, and that is usually operated at close to its natural resonance.
Various methods are known for regulating power. One method involves regulating the direct voltage or the direct current. Another method involves regulating power through frequency variation. Since the output network has a resonance frequency, the power in this output network can be increased by approximating the operating frequency to the resonance frequency and vice versa.
In so-called phase-shift methods, switching elements of the inverter are turned in for the maximum possible turn-on time, but the turn-on phases of the switching elements are offset relative to each other to allow the current to flow into the output network for only part of the duration of a half-wave. The phase-shift method presumes a full bridge in the inverter.
Another power regulating method not requiring a full bridge is the pulse-width modulation method (PWM), in which the switching elements of the inverter are turned on for only part of the possible turn-on time. As a result, only some of the power is relayed to the output network. The advantage of this method is that the frequency can remain stable (constant), which is a plus for many processes.
Ideally, no losses arise at the switching element(s) of the inverter, since the voltage at the switching element(s) is equal to zero, and the current is equal to zero in the turned-off state. The turn-on and turn-off steps can be problematic. An existing voltage can be disturbing when turning on the switching elements, since the switching element first remove the voltage, even though current is flowing and voltage is present until then.